This invention pertains to charge pumps for biasing a semiconductor substrate, well, or the like, and more particularly, to a method and apparatus for efficiently, and therefore more quickly, biasing the substrate or well to a final desired voltage.
Charge pumps are well known in the art as an on-chip voltage generator capable of providing a voltage more positive than the most positive external supply voltage and/or negative voltage in the absence of a negative external supply voltage. The advantages of charge pumps are also well known in the art such as providing a bias voltage for the substrate of an integrated circuit or N-type and P-type wells, or for providing greater output voltage swings, among other advantages.
Most charge pumps comprise some variation of the basic charge pump 10 shown in the schematic diagram of FIG. 1. The basic charge pump 10 configuration includes a ring oscillator 12 that provides a square wave or pulse train having voltage swings typically between ground and the most positive external power supply voltage, VCC. An invertor 14, buffer amplifier, or Schmitt trigger circuit may be used to sharpen the edges of the oscillating output signal of the ring oscillator 12. A capacitor 16 is discharged into the substrate 24 through diode-connected transistors 18 and 20. (Typically the drain and gate of a diode-connected transistor are coupled together to form the anode of a diode and the source forms the cathode of the diode.) Transistor 18 is coupled to the external power supply voltage, VCC, at terminal 22. When the ring oscillator 12 produces a voltage close to ground, circuit node 26 is approximately at the voltage of the power supply minus a transistor threshold voltage, VCC-VT. When the ring oscillator 12 produces a voltage close to VCC, the incremental charge on the capacitor 16 is delivered to the substrate 24. Capacitor 16 is prevented from discharging to any other circuit node by the reverse bias on diode-connected transistors 18 and 20.
In the charge pump 10, one pulse of current is delivered to the substrate 24 for every clock cycle of the ring oscillator. Therefore, charge pump 10 has an active half-cycle in which current is delivered to the substrate 24, and the charge asymptotically reaches the final desired voltage. However, charge pump 10 also has an inactive half-cycle in which capacitor 16 is precharged for the next active half-cycle. Although this inactive half-cycle is necessary to precharge the capacitor 16, no current is delivered to the substrate or well 24, which delays the attainment of the final desired voltage. In addition, the voltage across capacitor 16 is limited to VCC-VT due to the voltage drop across transistor 18.
In most integrated electronic circuits, such as a memory chip, it is desirable that the final voltage at the substrate or well be reached as quickly as possible. Proper device functions and attributes, such as the integrity of stored data, cannot be guaranteed until the substrate or well has reached the final value. Therefore, what is desired is a charge pump that more efficiently delivers current to a substrate or well in order to more quickly achieve a desired level of voltage bias.